Niobium-silicide alloys having a surface region of enhanced environmental-resistance, and related articles and processes

ABSTRACT

Niobium silicide articles are described. They include a surface region enriched with at least about 25 atom % germanium, which can enhance the properties of the article. Methods for preparing these articles are described as well. According to one method, an article is formed from a niobium silicide composite material which contains a selected amount of germanium. The article is then heat-treated under conditions sufficient to increase the level of germanium in the surface region to at least about 25 atom %, based on the total composition of the surface region. In another embodiment, a germanium-containing material is applied over a niobium-silicide article, and then diffused into the surface region of the article by way of a heat treatment.

BACKGROUND OF THE INVENTION

The present invention generally relates to refractory metalintermetallic composites. Some specific embodiments of the invention aredirected to the enhancement of various properties ofniobium-silicide-based articles which are very useful as turbine enginecomponents.

Turbines and other types of high-performance equipment are designed tooperate in a very demanding environment which always includeshigh-temperature exposure, and often includes high stress and highpressure. Superalloys based on elements like nickel or cobalt have oftenprovided the chemical and physical properties required for suchoperating conditions.

While the attributes of superalloys continue to ensure considerableinterest in such materials, new compositions have been developed to meetan ever-increasing threshold for high-temperature exposure. Prominentamong such materials are the refractory metal intermetallic composites(RMIC's). Examples include various niobium-silicide alloys. (The RMICmaterials may also include a variety of other elements, such astitanium, hafnium, aluminum, and chromium). These materials generallyhave much greater temperature capabilities than the current class ofsuperalloys. As an illustration, while many nickel-based superalloyshave an operating temperature limit of about 1100° C., many RMIC alloyshave an operating temperature in the range of about 1200° C.-1700° C.These temperature capabilities provide tremendous opportunities forfuture applications of the RMIC alloys. Moreover, the alloys areconsiderably lighter than many of the nickel-based superalloys.

While articles formed from the niobium-silicide alloys clearly possessvery attractive properties, continued improvement in certain areas wouldbe welcome in the art. As an example, great efforts are being made toimprove environmental protection, e.g., resistance to oxidation andcorrosion. Some of these efforts are necessary because niobium-silicidealloys can sometimes undergo rapid oxidation at temperatures above about1000° C. Under very demanding operating conditions, oxidation in thesurface region of the niobium-silicide article—even when the articleitself is covered by protective coatings—could ultimately damage thearticle.

It should thus be clear that niobium-silicide articles having improvedproperties would be very welcome in the art. In particular,niobium-silicide-based turbine components having improved environmentalresistance at elevated temperatures would be of considerable interest.The articles should also exhibit a general balance in other propertiesas well. For example, components such as turbine airfoils should also becharacterized as having good low-temperature toughness and good hightemperature strength. Moreover, it would also be desirable if thearticles could be made in a timely, cost-efficient manner, usingconventional manufacturing equipment.

BRIEF DESCRIPTION OF THE INVENTION

One embodiment of this invention is directed to a niobium silicidearticle which includes a surface region comprising at least about 25atom % germanium, based on the composition of the surface region.

Another embodiment relates to a method for preparing a niobium silicidearticle which includes a surface region enriched in germanium. Themethod comprises the following steps:

(a) forming an article from a refractory metal intermetallic compositematerial which comprises a metallic niobium-base phase, at least onemetal silicide phase, and at least about 10 atom % germanium, based ontotal atom percent of the composite material; and then

(b) heat-treating the article formed in step (a), under heatingconditions sufficient to increase the level of germanium in the surfaceregion to at least about 25 atom %, based on the total composition ofthe surface region.

An additional embodiment is directed to another method for preparingsuch a niobium silicide article. The alternative method comprises thefollowing steps:

(a) forming an article from a refractory metal intermetallic compositematerial which comprises a metallic niobium-base phase and at least onemetal silicide phase;

(b) applying a germanium-containing material (e.g., a coating) to asurface of the article formed in step (a); and then

(c) heat-treating the germanium-containing material and article, underconditions sufficient to cause at least a portion of the germanium todiffuse into a surface region of the article.

Other features and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM (scanning electron microscope) image of aniobium-silicide test coupon treated according to some embodiments ofthe present invention.

FIG. 2 is an EDS (energy dispersive X-Ray spectroscopy) representationof the image of FIG. 1, showing germanium content in the test coupon.

FIG. 3 is an EDS representation of the image of FIG. 1, showing oxygencontent in the test coupon.

FIG. 4 is a chart which demonstrates the effect of germanium levels onvarious sample alloys, after heat treatment steps.

FIG. 5 is another SEM image of a niobium-silicide test coupon treatedaccording to some embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The articles described herein are formed from niobium-silicide alloys,which are generally known in the art. Many suitable examples aredescribed in the following patents, which are all incorporated herein byreference: U.S. Pat. No. 5,833,773 (Bewlay et al); U.S. Pat. No.5,932,033 (Jackson et al); U.S. Pat. No. 6,409,848 (Bewlay et al); U.S.Pat. No. 6,419,765 (Jackson et al); and U.S. Pat. No. 6,676,381(Subramanian et al). The niobium-silicide alloys usually have amicrostructure comprising a metallic Nb-base phase and an intermetallicmetal silicide phase (e.g., Nb-silicide). However, they may include oneor more other phases as well. (As used herein, “alloy” is meant todescribe a solid or liquid mixture of two or more metals, or one or moremetals with one or more non-metallic elements).

In addition to niobium and silicon, the alloys usually include at leastone element selected from the group consisting of titanium (Ti), hafnium(Hf), chromium (Cr), and aluminum (Al). Ti and/or Hf are often preferredconstituents. A typical range for Ti is about 2 atom % to about 30 atom% (based on total atom % for the alloy material), and preferably, about12 atom % to about 25 atom %. A typical range for Hf is about 0.5 atom %to about 12 atom %, and preferably, about 2 atom % to about 8 atom %. Atypical range for Cr is about 0.1 atom % to about 25 atom %, andpreferably, about 2 atom % to about 20 atom %. A typical range for Al isabout 0.1 atom % to about 15 atom %, and preferably, about 0.1 atom % toabout 4 atom %.

The alloys frequently include other elements as well. Non-limitingexamples are nitrogen, molybdenum, yttrium, tantalum, rhenium,ruthenium, zirconium, iron, tungsten, germanium, carbon, and tin. Theparticular inclusion and amount for any of these elements will of coursedepend on a variety of factors, such as the desired properties for thefinal alloy product. As one illustration, the presence of molybdenum inalloys with the enriched germanium surface region may, under someconditions (though not all), adversely affect the oxidation-resistanceof the alloys.

The niobium silicide materials can be formed into useful articles by avariety of forming techniques. Casting is typically employed. Variousdetails regarding the casting of these refractory materials arewell-known in the art. Non-limiting examples of casting techniques aredescribed by Subramanian et al, in U.S. Pat. No. 6,676,381 (incorporatedherein by reference).

The niobium-silicide articles of this invention include a surface regionenriched in germanium. As further described below, the presence ofrelatively high levels of germanium in this region results insignificant improvements in some of the important characteristics of thearticle. The depth of the “surface region” will depend in part on thetype of article in use. As an example, the surface region for an articlewith relatively thin walls, e.g., a turbine airfoil, may be more shallowthan the surface region of an article which has a greater thickness.

In general, the “surface region” of the article (in terms of germaniumenrichment) is defined as the region which extends to a depth of nogreater than about 30% of the cross-sectional thickness of the article.In some specific embodiments, the depth is no greater than about 10% ofthe cross-sectional thickness. As used herein, “surface region” includesan affected region of the substrate, as well as any coating over theaffected region which is formed during the surface treatment process.The “affected region” is the region of the substrate in which diffusionhas occurred, according to some embodiments which are described below indetail.

As a non-limiting illustration in the case of a gas turbine blade, the“surface region” usually extends to a depth of about 50 microns into thesurface, and preferably, no greater than about 25 microns into thesurface. In the case of articles with greater cross-sectionalthicknesses, the surface region could extend to a depth of about 250microns. It should be understood that “surface region” refers to thesurface of the bulk alloy itself, treated according the invention. Inother words, the “surface” does not refer to oxidation layers which areformed on top of the bulk alloy, during one or more of the thermaltreatments described below.

In terms of the level of germanium, “surface enrichment” is meant todefine a concentration of at least about 25 atom % germanium, based onthe composition of the entire surface region. In general, this level ofgermanium is much higher than that which would be present in a typicalniobium-silicide alloy. In some specific embodiments, the level ofgermanium is at least about 40 atom %. In other embodiments, the levelof germanium is at least about 50 atom %. The maximum amount ofgermanium in the surface region is usually about 67 atom %, e.g., asestimated according to stoichiometric NbGe₂, the preferred phase forsome embodiments (as noted below). Those skilled in the art will be ableto select the most appropriate amount of germanium for a givensituation. As one illustration, higher amounts of germanium desirablyincrease the oxidation resistance of the article. However, an excessiveamount of germanium (especially if in free form, i.e., not in a phase,as described below) may lower the melting point of the surface region toa level which is not suitable for some end uses.

The germanium in the surface region is usually present in the form ofone or more phases. The types of phases present will depend on a varietyof factors, such as the elements present in the alloy substrate; therespective proportions of those elements; and the heating and processingconditions which are used to incorporate the germanium into the region(discussed below). Non-limiting examples of germanium-containing phaseswhich might be present are NbGe₂ (niobium digermanide); Nb₃Ge (e.g., theBeta phase); Nb₅Ge₃; TiGe₂, Ti₅Ge₃, Ti₆Ge₅, Ge—Hf, Ge—Si, Ge—Al, andGe—Cr. Ternary and higher-order derivatives of these phases are possibleas well. Moreover, one or more of the phases may be present as a solidsolution, which might contain individual elements as well. Furthermore,those skilled in the art understand that the elements in these phases donot have to be present in stoichiometric proportions.

In some specific embodiments, at least a portion of the germanium ispresent in the form of the niobium digermanide phase (NbGe₂). Thepresent inventors have discovered that the presence of this phase canconsiderably enhance oxygen diffusion barrier properties under someoperational conditions. In some embodiments, at least about 40% of thegermanium in the surface region is in the form of NbGe₂. In otherembodiments, at least about 50% of the germanium is in the form ofNbGe₂. In some especially preferred embodiments, at least about 70% ofgermanium in the surface region is present in the form of NbGe₂. (Itshould be understood that a niobium-germanide phase contains primarilythe two elements, although other elements may be present in solidsolution, in relatively minor amounts, e.g., a total of less than about15 atom %).

An additional embodiment of this invention relates to the formation ofarticles which include a surface region enriched in germanium. Accordingto one such method, a niobium-silicide article is formed by one of thetechniques mentioned previously, e.g., casting. The alloy used to formthe article comprises a metallic niobium-base phase, and at least onemetal silicide phase. (The alloy may include a variety of otherelements, as discussed previously).

The niobium-silicide alloy used in this embodiment further includesgermanium. When the article is heat-treated, the level of germanium inthe surface region increases to at least about 25 atom %, based on thetotal composition of the surface region. The inventors do not wish to bebound by any particular theory regarding the mechanism by which thegermanium level is increased. It is believed that, usually, at least aportion of the germanium in the bulk alloy migrates to the surfaceregion of the article. (For simplicity, this embodiment will sometimesbe referred to as the “migration” embodiment, although other mechanismsare suggested below).

The minimum amount of germanium present in the alloy is that whichresults in a surface region containing at least about 25 atom %germanium. (It should be understood that the enriched surface area wouldthus contain germanium which had migrated from the bulk alloy, i.e., thearea below the surface region, along with germanium which was initiallypresent in the surface region when the article was formed). When it isdesirable that the surface region contain an amount of germanium greaterthan about 25 atom %, the level of germanium in the bulk alloy can bemodified accordingly. For example, the bulk alloy can be formulated tocomprise (i.e., prior to any heat treatment) at least about 25 atom %germanium, and more often, at least about 35 atom % germanium.

The heat treatment employed in this embodiment will depend on manyfactors. They include: the specific composition of the alloy; themicrostructure of the alloy; the amount of germanium enrichment desired;the depth of the bulk alloy (which in some cases appears to serve as areservoir of germanium); the heat-treatment environment (e.g., heatingatmosphere; type of heating cycles); and the heating mechanism. Anotherfactor influencing the selected heat treatment relates to the rate ofoxide growth. As an example, if the heat treatment is carried out at toohigh a temperature and/or for too long a period of time, the overlyingoxide layer may grow too fast, which may in turn prevent the formationof the enriched surface region. For a typical niobium-silicide alloy,the heat treatment will usually be carried out at a temperature in therange of about 600° C. to about 1400° C. In some specific embodiments,the heat treatment is carried out at a temperature in the range of about1000° C. to about 1250° C.

The heat treatment can be carried by using various types of equipment,e.g., employing a suitable convection or conduction mechanism. As anexample, a standard furnace could be used. An oxidizing atmosphere suchas air is the most preferred heating environment for this embodiment.

Heating times will also depend on many of the factors set forth above,including the particular heating equipment employed. In the case ofniobium-silicide alloys having approximate dimensions of 1 cm×1 cm×1 cm,the heating time will usually be in the range of about 30 minutes toabout 200 hours. More often, the heating time will be in the range ofabout 1 hour to about 100 hours. Typically, longer heating time periodscan compensate for lower heating temperatures (within the general rangesnoted above), while higher temperatures can compensate for shorter timeperiods. (In some commercial applications, the heat treatment time istypically no greater than about 50 hours). The most appropriate heatingregimen can readily be determined by a series of tests, to determinewhich parameters provide the desired amount of germanium enrichment tothe surface region. As further described in the examples, the amount ofgermanium present in that region can accurately be determined by varioustechniques, e.g., X-ray diffraction, electron microprobe techniques; EDS(energy dispersive X-Ray spectroscopy); WDS (wavelength dispersive X-Rayspectroscopy); and wet chemical analysis. It should also be understoodthat a portion of the heat treatment can effectively occur when thearticle is put into service, e.g., a gas turbine component operatingunder normal conditions.

The heat treatment in this embodiment also results in the formation ofone or more oxide layers over the enriched surface region. The oxidelayer can be referred to as an oxide “scale”. It is formed primarilywhen the heat treatment is carried out in an oxidizing atmosphere. Theoxide scale may contain different phases, depending in part on thecontent of the bulk alloy, along with the particular heat treatmentconditions employed. Moreover, when the oxide scale is in the form ofmultiple layers, each layer may primarily contain one phase, e.g., aniobium-rich oxide phase or a silicon-rich oxide phase. The thickness ofthe layer will depend in part on the other factors described herein(especially heat treatment time and temperature; and bulk alloycomposition). Usually, the layer will have a thickness of about 5microns to about 250 microns.

In some embodiments, the oxide scale can remain on the article during agiven end use. However, in other situations, it is desirable to removethe oxide scale. Removal can be undertaken by various techniques, e.g.,abrasion with a suitable media such as glass beads; polishing, grinding,and the like.

As mentioned above, it appears that at least some of the germaniumenrichment occurs by way of a migration mechanism. However, germaniumenrichment may also be occurring due to other mechanisms. For example,the increase in germanium as a proportion of the constituents in thesurface region can occur because other constituents in that region, suchas niobium, silicon, and titanium, are leaving that region by way oftransformation into the oxide scale discussed previously.

According to another embodiment, the germanium required for surfaceenrichment of the niobium-silicide article can be diffused from amaterial (e.g., a coating) over the surface of the article. Manydifferent techniques can be used to carry out this technique. As anexample, the germanium, or a germanium-containing compound or mixture,could be deposited on the substrate surface by using a slurrycomposition. According to this technique, the germanium could be used inthe form of the metal itself; in the form of an alloy, or as a compoundor metal mixture which melts at a temperature below the melting point ofthe substrate. The alloy, compound, or metal mixture may include otherbeneficial elements, such as chromium, niobium, aluminum, and the like.(Intermetallic compounds of germanium are usually not preferred, becausetheir melting points may be too high. However, intermetallics may formin situ during the subsequent heat treatments, and this occurrence isdesired). The amount of germanium selected for the slurry will depend inlarge part on the amount of germanium desired for the enriched surfaceregion of the substrate.

Metal-containing slurry compositions are well-known in the art, as aretheir methods for preparation. Usually, the germanium or germanium alloyin the slurry will have an average particle size in the range of about0.5 micron to about 50 microns, and more often, in the range of about 1micron to about 10 microns. (Moreover, the alloy particles arepreferably spherical or substantially spherical when the material is tobe deposited by a spray technique). The slurry can be aqueous ororganic, depending on various factors, such as its specific content, andthe manner in which it will be applied to the article. Furthermore, theslurry can include various other ingredients, such as stabilizers (e.g.,organic stabilizers), which chemically stabilize the slurryconstituents. Stabilization of the slurry can be important, e.g., whenvery fine metal particles are incorporated therein. Additives whichimprove the wettability of the slurry to the substrate surface are alsoused when appropriate.

The slurry can contain various other ingredients as well. Many of theseare known in the art to those involved in slurry preparations. Slurriesare generally described in “Kirk-Othmer's Encyclopedia of ChemicalTechnology”, 3rd Edition, Vol. 15, p. 257 (1981), and in the 4thEdition, Vol. 5, pp. 615-617 (1993), as well as in U.S. Pat. Nos.5,759,932 and 5,043,378. Each of these references is incorporated hereinby reference. A good quality slurry is usually well-dispersed and freeof air bubbles and foaming. It typically has a high specific gravity andgood rheological properties adjusted in accordance with the requirementsfor the particular technique used to apply the slurry to the substrate.Moreover, the solid particle settling rate in the slurry should be aslow as possible, or should be capable of being controlled, e.g., bystirring.

The slurry can be applied to the surface by many different techniques.For example, it can be slip-cast, brush-painted, dipped, sprayed,poured, rolled, or spun-coated onto the substrate surface. Spray-coatingis often the easiest way to apply the slurry coating to substrates whichhave complex geometric shapes, such as turbine airfoils. The viscosityof the coating can be readily adjusted for spraying, by varying theamount of liquid carrier used. Spraying equipment is well-known in theart. Any spray gun should be suitable, including manual or automatedspray gun models; air-spray and gravity-fed models, and the like.Adjustment in various spray gun settings (e.g., for pressure and slurryvolume) can readily be made to satisfy the needs of a specificslurry-spraying operation. The slurry can be applied as one layer, ormultiple layers.

After the slurry coating has been applied to the surface of the article,it is heat-treated. The heat treatment conditions are those which aresufficient to cause at least a portion of the germanium in the slurry todiffuse into the surface region of the article. As in the otherembodiments, the heat treatment can be carried out by using varioustypes of equipment, e.g., a standard furnace. In this embodiment, theheat treatment is carried out in either a vacuum or an inert atmosphere.A vacuum is preferred.

Heating times and temperatures for this embodiment will also depend onsome of the factors set forth above, including the particular heatingequipment employed. Usually, the heating temperature is based on themelting temperature (T_(m)) of the germanium-containing material (e.g.,element/alloy/compound) in the slurry. Thus, the heating temperature isusually in the range of about (0.8)T_(m) to about (1.5)T_(m) of thematerial. In some preferred embodiments, the heating temperature is inthe range of about (1.2)T_(m) to about (1.4)T_(m). Thus, if elementalgermanium (with a melting temperature of 937° C.) were used in theslurry, the broader range would be about 750° C. to about 1405° C. (Itshould be understood, however, that the temperature used should notexceed the melting point of the substrate).

Heating times will usually be in the range of about 10 minutes to about10 hours. More often, the heating time will be in the range of about 30minutes to about 90 minutes. As in the other embodiments, longer heatingtime periods can compensate for lower heating temperatures, while highertemperatures can compensate for shorter time periods. Moreover, theamount of germanium which is incorporated into the surface region can beascertained by the various techniques discussed previously, so that theoptimal diffusion conditions can be determined.

Other methods for applying a germanium-containing coating to the surfaceof an article are also possible. Non-limiting examples include plasmadeposition (e.g., cathodic arc deposition; vacuum plasma spraying (VPS);high velocity oxy-fuel (HVOF) techniques; and air plasma spray (APS));physical vapor deposition (PVD); chemical vapor deposition (CVD); packdeposition techniques; and sputtering. Those of ordinary skill in theart are familiar with details regarding each of these techniques. It isalso understood that the germanium material would be used in a formwhich is compatible with the specific deposition technique. As anexample, the thermal spray techniques (e.g., VPS, HVOF, and APS) wouldusually employ the germanium (or germanium alloy) in powdered form. Theheating techniques to permit diffusion of the germanium can be adjustedto suit the particular deposition technique. (Those skilled in the artwould understand that the heat treatment during any of the processesdescribed herein may result in relatively minor changes in the thicknessof the overall article, e.g., due to elemental interdiffusion and thelike).

As mentioned above, the germanium can also be diffused into the articlesurface by a “pack” process. Details regarding pack techniques (alsoreferred to as “pack cementation” techniques) are known in the art anddescribed, for example, in U.S. Pat. No. 6,110,262, which isincorporated herein by reference. As a general example, the articlecould be embedded in a powder pack containing germanium (in metal-,alloy-, or compound-form). The pack also contains an activator—typicallyan ammonium or alkali metal halide carrier—and an inert filler.

Once embedded, the article is usually enclosed in a sealed chamber, andthen heated to a temperature similar to the diffusion temperaturesmentioned above. Under these conditions, the halide activatordissociates, and reacts with the germanium from the metallic source.This reaction produces gaseous germanium halide species, which canmigrate into the surface region of the article. The germanium-richvapors are reduced by the metals at the alloy surface, to formintermetallic compounds which provide the enriched germanium content.

When the germanium-enriched surface region is formed by the diffusiontechniques, the oxide scale described previously is not formed. (A verysmall, incidental amount may in some cases be unintentionally formed,e.g., when there is a variation in process steps). The substantialabsence of an oxide layer represents a significant advantage for theformation of the germanium-enriched layer by the diffusion technique.

As mentioned above, the “surface region” includes an affected region ofthe substrate, as well as any overlying coating related to the diffusionprocess. Thus, in this embodiment, the affected region is the region inwhich diffusion has occurred, i.e., as contrasted with the underlyingbulk alloy portion which is not affected at all by the diffusiontreatment. The overlying coating in this embodiment is the non-diffusedportion of the slurry or similar material that may sometimes remain onthe surface after treatment is complete. (It is thought that thiscoating may remain on the surface more frequently when a solid statematerial is used as the treatment agent, as compared to a liquid-statematerial). In some instances, it may be desirable to remove the coatingfrom the surface, e.g., using the techniques described above. However,in other cases the coating can remain, and enhance the overallproperties of the article.

The germanium-enriched surface region can provide an improved degree ofoxidation resistance to the niobium-silicide articles, over a range ofaggressive environments. A variety of articles can benefit from thisimportant characteristic. Many of them are components for turbines,e.g., land-based turbines, marine turbines, and aeronautical turbines.Specific, non-limiting examples of the turbine components are buckets,nozzles, blades, rotors, vanes, stators, shrouds, combustors, andblisks. Non-turbine applications are also possible. (In some preferredembodiments, the niobium-silicide articles, as ready for use, include agermanium level in the bulk region (below the surface region) of nogreater than about 10 atom %).

Embodiments of this invention are useful for providing additionalenvironmental resistance to articles which have internal surfaces, e.g.,holes, cavities, depressions, passageways, and the like. As an example,a turbine blade formed from a niobium-silicide alloy may include anumber of cooling holes and passageways, e.g., for channeling bypass airfrom the compressor of the turbine. In preferred embodiments, thediffusion process for internal surfaces can be carried out with a packcementation technique. Alternatively, diffusion could be carried out bydirecting a slurry (e.g., by pumping) through the passageways. (Careshould be taken to avoid blockage of the passageways). By using thesetechniques, the alloy surface of the hole or passageway can be enhancedwith a germanium-based phase. In this manner, the interior surface—whichis often difficult to efficiently coat and protect by other methods—canreceive an added measure of environmental resistance, e.g., resistanceto the harmful effects of oxidation.

In some embodiments, the surface region is compositionally graded, inregard to the level of germanium. For example, the concentration ofgermanium can vary through the depth of the region. (Some gradation mayalso be present in the bulk alloy). Preferably, however, the total levelof germanium within the enriched region remains at the level describedpreviously, i.e., at least about 25 atom %. The gradation may besubstantially continuous, but this does not always have to be the case.

In the situation where enrichment is achieved by the migrationmechanism, i.e., from within the bulk alloy, gradation may usually beevidenced by a gradual decline in germanium concentration in the upwarddirection, i.e., away from the substrate. In the situation whereenrichment is achieved by diffusion from a layer deposited over thesubstrate, gradation may usually be evidenced by a gradual decline ingermanium concentration in the downward direction, i.e., toward thesubstrate. There are advantages to gradation in some situations. Forexample, gradation of the germanium level can result in a gradation of“thermophysical properties”, i.e., the physical characteristics of amaterial at elevated temperatures. Examples of those properties are thecoefficient of thermal expansion (CTE), thermal conductivity, andstrength.

In addition to its function as an oxygen-barrier layer, the enrichedgermanium surface region can function as a bond layer for an overlyingprotective coating, e.g., a ceramic overcoat. An example of such anoverlying coating is a thermal barrier coating (TBC). TBC's are oftenformed from materials like zirconia, stabilized zirconia (e.g.,yttria-stabilized), zircon, mullite, and combinations thereof; as wellas other refractory materials having similar properties. These coatingsare well-known in the art and described, for example, in a patent issuedto Zhao et al, U.S. Pat. No. 6,521,356, which is incorporated herein byreference.

TBC's and other types of overcoats can be applied by many conventionaltechniques. Some of the techniques were listed previously, such as PVD.The thickness of the TBC can vary greatly, depending on many factors.Usually, the coating has a thickness in the range of about 10 microns toabout 600 microns. (In those cases where the enriched germanium regionhas been formed by heating the bulk alloy, the oxide scale formed on topof the enriched region should usually be removed, prior to deposition ofthe TBC or other top coating).

In other embodiments, a separate protective coating can be applied overthe article having the germanium-enriched surface region. Thisprotective coating could serve as the sole overlying coating (i.e., thetop layer of the article, providing further oxidation resistance), or itcould function as a bond coat for a TBC or other protective topcoat. Inthose instances in which an oxide scale is present over the enrichedsurface region, it may sometimes be desirable to remove the scale beforeapplication of this protective coating.

Useful protective coatings of this type (i.e., serving as bond coatingsor oxidation-resistant coatings) often comprise silicon, titanium,chromium, and niobium, as described in U.S. Pat. No. 6,521,356. Somecompositions of this type contain about 43 to about 67 atom % silicon;between about 2 and about 25 atom % titanium; between about 1 and about25 atom % chromium; and a balance of niobium. Many other constituentscan be incorporated into the compositions. Non-limiting examples includeboron, tin, iron, germanium, hafnium, tantalum, aluminum, tungsten, andmolybdenum.

As another example, coatings based on chromium, ruthenium, and aluminumcan also be used to effectively protect niobium silicide components.Examples of this type can be found in a patent issued to M. Jackson,U.S. Pat. No. 4,980,244, which is incorporated herein by reference. Manyof these coatings comprise about 32 atom % to about 62 atom % chromium;about 19 atom % to about 34 atom % ruthenium; and about 19 atom % toabout 34 atom % aluminum. They may also include one or more otherelements, such as yttrium, iron, nickel, and cobalt.

Other coatings which promote oxidation resistance are based onsilicon-iron-chromium alloys. Specific examples are described in U.S.Pat. No. 5,721,061 (Jackson et al), which is incorporated herein byreference. For example, some embodiments contemplate materials whichcomprise (in weight percent) about 26% to about 32% iron, and about 24%to about 30% chromium; with the balance being silicon. In some cases,these types of coatings are heat-treated after being applied over thesubstrate (e.g., at about 1250° C. to about 1400° C.). As described byJackson et al, the heat treatment (which could sometimes be combinedwith or satisfied by other heat treatments on the article) results in acoating which comprises an outer layer and an interaction layer betweenthe outer layer and the substrate material. The interaction layerincludes one or more metallic-silicide phases which further enhance theprotective capabilities of the overall coating.

The thickness of the protective coating can vary greatly, depending onmany factors like those described above, and also depending on whetheror not a TBC or other top coat is to also be used. In some specificembodiments, the coating has a thickness between about 10 microns andabout 400 microns. Moreover, the coating can be applied by a variety oftechniques, as also described above, such as APS, HVOF, slurrydeposition, and the like.

EXAMPLES

The examples which follow are merely illustrative, and should not beconstrued to be any sort of limitation on the scope of the claimedinvention.

Example 1

A niobium-silicide sample was prepared by dry-mixing a composition withthe following nominal constituents: 38.7 atom % Nb, 2.0 atom % Hf. 18.4atom % Ti, 0.9 atom % Al, 2.7 atom % Cr, 12.2 atom % Si, and 1.9 atom %Sn. The sample also contained 23.2 atom % Ge. The composition wasarc-melted, to prepare an alloy sample in the shape of a disc. A testcoupon was cut from the sample, and had approximate dimensions of 1 cm×1cm×1 cm. A surface of the coupon was polished to remove any dirt andimpurities. The coupon was then placed in a conventional box furnace,and heated in an air atmosphere for 100 hours, at a temperature of 1150°C.

FIG. 1 is a cross-sectional representation of the test coupon (obtainedby SEM (scanning electron microscope) after removal from the boxfurnace. As depicted in the figure, article 10 includes a bulk alloyportion 12, i.e., the main portion of the alloy body. The bulk alloyportion, both before and after the heat treatment, was primarily made upof an (Nb,Ti)₅(Si,Ge)₃ phase.

Surface region 14, having an average depth of about 50 microns, isdisposed over bulk alloy portion 12. The surface region was enriched ingermanium, and primarily comprised the niobium digermanide (NbGe₂)phase. As described above, surface region 14 was formed during the heattreatment, presumably by the upward migration of germanium from the bulkalloy portion 12. A sectional analysis of the composition of the niobiumdigermanide phase itself, via microprobe, indicated a composition ofapproximately 28 atom % Nb, 64.5 atom % Ge, 2 atom % silicon, 5 atom %Ti, and 0.5 atom % Hf.

The heat treatment in the oxidizing atmosphere also resulted in theformation of an oxide layer over the surface region. The oxide layer,i.e., oxide scale 16, primarily contained niobium-rich oxide phases andsilicon-rich oxide phases which appeared to be somewhat layered. Thethickness of the overall oxide layer was about 250 microns.

FIG. 2 is an EDS representation of the image of FIG. 1, in which thespectroscopic scan is programmed to represent germanium levels, by colordifferentiation. The enriched surface region 14 is disposed over bulkalloy portion 12. (It should be noted that the EDS instrument provides avery clear display of enriched layer 14, in color).

FIG. 3 is another EDS representation of the image of FIG. 1. In thisinstance, the spectroscopic scan is programmed to represent oxygencontent, by color differentiation. Region 18 is the bulk alloy, on whichthe germanium-enriched surface layer has been formed (not fully visiblein this image). Region 16 is the oxide layer formed over the enrichedsurface layer. It is clear from the figure that there has not been anyoxygen penetration into the surface layer or the bulk alloy, indicatingthat the integrity of the alloy can be maintained under thesetemperature and oxidation conditions.

Example 2

Niobium-silicide test samples were prepared by arc-melting, according tothe general procedure described in Example 1. The specific compositionof each sample is indicated in FIG. 4. Each of the samples was subjectedto oxidation testing at temperatures of 1150° C. and 1000° C., for 100hours. Sample 1 was based on embodiments of the present invention. Thesample contained 23.2 atom % germanium, which migrated to the surface toform the enriched layer during the oxidizing heat treatment. Samples 2and 3 were comparative alloys. They contained relatively low levels ofgermanium, and were outside of the scope of this invention.

The chart in FIG. 4 depicts the weight-change in the sample, after theparticular heat-treatment tests. As those familiar with these testsunderstand, weight loss is often an indication of oxide spallation.(Weight loss can also occur due to the evaporation of gaseous oxides).Oxide spallation is usually initiated by the penetration of oxygenand/or oxygen compounds into at least the surface region of the article,resulting in the loss of non-adherent (or loosely-adherent) materialfrom the article. An excessive amount of spalling can lead to seriousdegradation of the article.

After the 1150°/100 hour thermal exposure test, comparative sample 2,which contained a relatively low germanium level, and did not have agermanium-enriched surface region, spalled badly, resulting in a largeweight loss. Comparative sample 3, which also did not include agermanium-enriched surface region, spalled to some degree, with aconsequent, significant weight loss. It is believed that the spallationoccurred because of the penetration of oxygen, as discussed previously.Moreover, although only a theory, it appears that sample 2 degraded morethan sample 3, because of the presence of significant amounts ofmolybdenum (5 atom %).

In contrast to samples 2 and 3, sample 1 showed no weight loss, and infact showed a weight gain. This sample was within the scope of thepresent invention, and contained the enriched germanium surface region.The weight gain is an indication that the sample did absorb some oxygen,but the resulting microstructure in the general surface region remainedstrongly adherent to the underlying bulk alloy. Sample 1 represents anarticle which provides much greater resistance to oxidation and theother accompanying degrading effects, as compared to samples 2 and 3.

After the 1000° C./100 hour thermal exposure test, comparative sample 2did not show a weight loss under these conditions, instead exhibiting aweight gain of 43 mg/cm². However, the weight gain in this instance wasnot as high as sample 1, which was based on the present invention (i.e.,49 mg/cm² for sample 1). Sample 3 did show a weight loss (−17 mg/cm²),indicating some degradation of the alloy article, although the weightloss was not as great as in the 1150° C. test.

Example 3

In this example, the germanium-enriched layer was formed by way of thediffusion technique described previously. A germanium (Ge) slurry wascreated by mixing Ge metal with an organic binder and carrier. Thebinder for this example was a Remet® product called Ethyl Silicate 40.The carrier was ethyl alcohol, which also functioned as an agent toadjust the viscosity of the slurry. Ge metal powder (purchased from AlfaAesar), sieved to a −325 mesh, was combined with the binder and carrierin the following concentration:

50.0 g Ge

27.5 g Remet® Ethyl Silicate 40

27.5 g Ethyl alcohol

The mixture was sealed in a container and mixed via a paint shaker for30 minutes, prior to being loaded into a gravity-fed spray gun. Themixture was air-sprayed on a niobium-silicide substrate surface, using aconventional DeVilbiss spray gun. The slurry was allowed to air-dry onthe coupon, and then a second layer was sprayed over the first. Thesecond layer of slurry was allowed to air-dry on the substrate. Afterbeing air-dried, the coated coupon was cured in an oven, according tothis heating regimen: 80° C. for 60 minutes, followed by 120° C. for 30minutes, followed by 220° C. for 60 minutes.

The coated coupon was then diffusion heat-treated in a vacuum oven, at atemperature of about 1000° C. for 60 minutes. FIG. 5 is across-sectional representation of the resulting sample, obtained by SEM.As depicted in the figure, article 20 includes a non-diffused alloyregion 22. An enriched region 24 is present over region 22. Enrichedregion 24 had a thickness of about 10-25 microns, and had aconcentration of germanium within the acceptable ranges set forth above.No evidence of coating spallation was seen. The top of the coating had afriable layer 26 that is easily removed via a light glass bead-blasting.

Various embodiments of this invention have been described in rather fulldetail. However, it should be understood that such detail need not bestrictly adhered to, and that various changes and modifications maysuggest themselves to one skilled in the art, all falling within thescope of the invention as defined by the appended claims.

1. A niobium silicide article which includes a surface region comprising at least about 25 atom % germanium, based on the composition of the surface region.
 2. The article of claim 1, wherein the surface region comprises at least about 40 atom % germanium.
 3. The article of claim 1, wherein the amount of germanium in the surface region is in the range of about 25 atom % to about 67 atom %.
 4. The article of claim 1, wherein at least a portion of the germanium in the surface region is in the form of a niobium germanide phase.
 5. The article of claim 4, wherein the niobium germanide phase is niobium digermanide (NbGe₂).
 6. The article of claim 5, wherein at least about 40% of the germanium in the surface region is in the form of the niobium digermanide phase.
 7. The article of claim 6, wherein at least about 50% of the germanium in the surface region is in the form of the niobium digermanide phase.
 8. The article of claim 1, wherein the surface region extends to a depth of about 30% of the cross-sectional thickness of the article.
 9. The article of claim 1, wherein the surface region extends to a depth of about 50 microns.
 10. The article of claim 1, comprising a bulk alloy region below the surface region, wherein the bulk alloy comprises a metallic niobium-base phase and at least one metal silicide phase.
 11. The article of claim 10, wherein the bulk alloy region further comprises titanium and at least one element selected from the group consisting of rhenium and ruthenium.
 12. The article of claim 10, wherein the bulk alloy further comprises titanium and at least one element selected from the group consisting of hafnium, chromium, and aluminum.
 13. The article of claim 11, wherein the bulk alloy further comprises at least one element selected from the group consisting of silicon, zirconium, tin, tungsten, and carbon.
 14. The article of claim 1, wherein at least one oxide layer is disposed over the surface region.
 15. The article of claim 14, wherein the oxide layer is formed by a heat treatment which is carried out to form the germanium-containing surface region.
 16. The article of 1, wherein the germanium in the surface region is compositionally graded.
 17. The article of claim 1, wherein at least one protective coating is disposed over the surface region.
 18. The article of claim 17, wherein the protective coating is an oxidation-resistant coating.
 19. The article of claim 18, further comprising a thermal barrier coating disposed over the oxidation-resistant coating.
 20. The article of claim 1, in the form of a turbine engine component.
 21. The article of claim 20, wherein the turbine engine component is selected from the group consisting of turbine buckets, nozzles, blades, rotors, vanes, stators, shrouds, combustors, and blisks.
 22. A turbine component, formed at least in part from a niobium-silicide alloy which comprises niobium (Nb), silicon (Si); and at least one element selected from the group consisting of titanium (Ti), hafnium (Hf), chromium (Cr), and aluminum (Al); wherein a surface region of the niobium-silicide alloy comprises at least about 25 atom % germanium, and at least a portion of the germanium in the surface region is in the form of the niobium digermanide phase.
 23. The turbine component of claim 22, further comprising at least one protective coating over the surface region of the alloy.
 24. The turbine component of claim 22, further comprising an oxidation-resistant coating over the surface region of the alloy; and a yttria-stabilized zirconia thermal barrier coating disposed over the oxidation-resistant coating.
 25. The turbine component of claim 22, comprising at least one hole or passageway in a niobium-silicide portion of the component, wherein the interior alloy surfaces of the hole or passageway also comprise at least about 25 atom % germanium.
 26. A method for preparing a niobium silicide article which includes a surface region enriched in germanium, comprising the following steps: (a) forming an article from a refractory metal intermetallic composite material which comprises a metallic niobium-base phase, at least one metal silicide phase, and at least about 10 atom % germanium, based on total atom percent of the composite material; and then (b) heat-treating the article formed in step (a), under heating conditions sufficient to increase the level of germanium in the surface region to at least about 25 atom %, based on the total composition of the surface region.
 27. The method of claim 26, wherein at least a portion of the increase in the level of germanium in the surface region is caused by the migration of germanium from the article formed in step (a), up to the surface region.
 28. The method of claim 26, wherein the surface region extends to a depth of about 50 microns.
 29. The method of claim 26, wherein the heat treatment is carried out in an oxidizing atmosphere.
 30. The method of claim 29, wherein the heat treatment is carried out at a temperature in the range of about 600° C. to about 1400° C.
 31. The method of claim 26, wherein the heat treatment causes the formation of the niobium digermanide (NbGe₂) phase in the surface region.
 32. The method of claim 31, wherein at least about 40% of the germanium in the surface region is in the form of the niobium digermanide phase, after the heat treatment.
 33. A method for preparing a niobium silicide article which includes a surface region enriched in germanium, comprising the following steps: (a) forming an article from a refractory metal intermetallic composite material which comprises a metallic niobium-base phase and at least one metal silicide phase; (b) applying a germanium-containing material to a surface of the article formed in step (a); and then (c) heat-treating the germanium-containing material and article, under conditions sufficient to cause at least a portion of the germanium to diffuse into the surface region of the article.
 34. The method of claim 33, wherein the surface region extends to a depth of about 30% of the cross-sectional thickness of the article.
 35. The method of claim 33, wherein the germanium-containing material comprises elemental germanium, or a germanium-containing compound or mixture.
 36. The method of claim 33, wherein the germanium-containing material is applied in the form of a slurry.
 37. The method of claim 36, wherein the slurry is applied by a technique selected from the group consisting of slip-casting, brushing, dipping, spraying, pouring, roll-coating, and spin-coating.
 38. The method of claim 33, wherein the germanium-containing material is applied to the surface of the article by a technique selected from the group consisting of plasma deposition; physical vapor deposition (PVD); chemical vapor deposition (CVD); sputtering; and pack processes.
 39. The method of claim 33, wherein the heat treatment of step (c) is carried out in a vacuum or an inert atmosphere.
 40. The method of claim 39, wherein the heat treatment is carried out at a temperature in the range of about (0.8)T_(m) to about (1.5)T_(m) of the germanium-containing material, where “T_(m)” represents the melting temperature. 